The present invention relates to high resolution intravascular imaging and more particularly to intravascular ultrasound imaging and techniques for enhancing image quality.
In intraluminal or intravascular ultrasound (also referred to as "IVUS") imaging, the production of high resolution images of vessel wall structures requires imaging at high ultrasound frequencies. In some types of intraluminal systems, an ultrasonic unidirectional exciter/detector within a catheter probe positioned within a blood vessel is used to acquire signal data from echoes of the emitted ultrasonic energy off the interior of the blood vessel. Specifically, vectors are created by directing focused ultrasonic pressure waves radially from a transducer in a catheter and collecting echoes at the same transducer from the target area. A plurality of radial vectors from the rotated transducer comprises an image frame. A signal processor performs image processing (e.g., stabilization of a moving image, temporal filtering for blood speckle, and other image enhancement techniques) on the acquired data in order to provide a display of the corrected and filtered intravascular image on a raster-scan display monitor.
It is desirable to provide imaging over a broad range of frequencies (e.g., 5 Megahertz (MHz) to 50 MHz), especially higher ultrasonic frequencies in some applications. However, the backscatter from blood cells in such an image is a significant problem in high frequency intraluminal ultrasound imaging, since the scattering of ultrasound from blood cells is proportional to the fourth power of the frequency such that the higher the ultrasound frequency the more pronounced is the backscatter from blood. As a result, echoes from blood molecules degrade the lumen-to-vessel wall contrast, which is undesirable since there is a need to define the blood/tissue boundary in order to ascertain the degree of narrowing of the vessel and to determine the spatial extent of the plaque. Therefore, echoes in the ultrasound image due to backscatter from blood (the irregular pattern of backscatter from blood is referred to as "blood speckle") must be detected in order to provide an enhanced image display. Once detected, the blood speckle may be removed or suppressed to a level at which wall structures can be distinguished from blood, distinguished by providing a different display color for the blood, and/or used to better delineate the blood/tissue interface.
Various techniques have been used in intravascular ultrasound imaging for detecting blood speckle in the image. These techniques are not always effective in distinguishing between blood and tissue, because they are based on key assumptions which are not always true. Some techniques rely on the assumption that the energy scattering strength from blood is low in comparison to the scattering strength from tissue, in order to distinguish between blood and tissue. Other techniques rely on the assumption that the blood moves much faster compared to the tissue and thus has a different Doppler signal than the tissue. In reality, however, such assumptions may be violated. In particular, the energy scattering from blood can sometimes be equally as bright as the scattering from tissue, and/or blood may sometimes move with very low velocity or not be moving at all. Although generally effective, these techniques may not be so effective in situations when these assumptions are not valid.
From the above, it can be seen that alternative or supplementary methods and apparatus are needed for detecting blood speckle to allow a display of intraluminal ultrasound images to be free of or to distinctly identify blood-induced echoes.